The inspector general for the Department of Health and Human Services said in a report (pdf) that Medicare paid $54 million for ambulance rides that were unnecessary or not covered by the agency in the first six months of 2012. Of the nearly 16,000 ambulance services in the United States used to transport Medicare patients, one in five were found to have one or more questionable bills, according to investigators.

Ambulance service is covered when a patient is being transported for a medical service covered by Medicare. The inspector general found that $30 million was spent transporting patients for whom a corresponding covered service could not be found. Another $24 million was spent on service to places, such as doctors’ offices, to which ambulance rides are not covered.

Most of the incorrect billing is concentrated in four metropolitan areas: Philadelphia, Los Angeles, New York and Houston, according to The Washington Post. Not surprisingly, the inspector general also found signs of overbilling via inflated mileage charges. In those cases, the transport distance in urban areas that was cited in billing—34 miles—was triple the average distance for Medicare patients from medical facilities.

Medicare has begun to fight overcharging for services, blocking new ambulance companies from participating in Medicare in Houston and Philadelphia and requiring authorization for repetitive non-emergency trips in Pennsylvania, New Jersey and South Carolina, according to the Post.

Medicare spent $5.8 billion on ambulance services in 2012, double what it paid in 2003, according to the report.

The inspector general recommended that Medicare require certification of ambulance claims by doctors, monitor ambulance billing and improve its claims process.